When in Preview Mode there will be a little Black X Icon over on the Top Right
of your screen. It may be just out of shot if you are on a smaller screen computer
or on a Laptop.
But there is an easy fix for this....
To Exit the Preview Mode just Zoom Out for a second, then the X Icon
will come into view and you will be able to Exit the Preview Mode and go back to
the Banner Ad Canvas Area.
This article will give you detailed instructions on how to do this :